The present invention relates to a pneumatic run flat tire for motorcycles. More particularly, the present invention relates to a pneumatic run flat tire for motorcycles, which tire is provided with reinforced sidewall portions having such a high rigidity that, even when the tire is punctured while running, the tire can support the load of the motorcycle and maintain a high durability and a high steering stability thereof when the motorcycle is not only running straight ahead but, also, making a turn or running in a zigzag movement, and which tire can be easily manufactured at a high rate of productivity.
In conventional pneumatic tires which are usually used for four-wheel and two-wheel vehicles, an air chamber formed inside the tire is inflated by compressed air and the load of the vehicle is supported by the inflated air chamber having a high elasticity. Therefore, the sidewall portions of the conventional tires have a relatively low rigidity. In this type of conventional tires, when the compressed air leaks from the air chamber and the pneumatic pressure of the air chamber becomes equal to atmospheric pressure, the tire will collapse at the portion thereof which is in contact with the road surface under the weight of the vehicle in such a manner that each of the sidewall portions of the tire is folded on itself so as to project outwards from the tire-mounting rim. In this condition, the collapsed tire cannot absorb the lateral force applied to the sidewall portions of the tire when the vehicle runs in a zigzag movement or makes a turn. The lateral force applied to the tire when the vehicle runs in a zigzag movement or makes a turn, includes a cornering centrifugal force which is produced by imparting a slip angle to the tire relative to the longitudinal center line of the vehicle during turning and also includes a cambering centrifugal force which is produced by inclining the tire relative to the vertical direction in accordance with a camber angle. Therefore, the steering stability of the vehicle is significantly reduced. Also, the collapse of the tire results in an irregular rotation of the tire. The irregular rotation causes the vehicle body to produce undesirable vibrations and, thus, remarkably reduces the handling stability of the vehicle. When the tire is subjected to such irregular rotation at a high speed for a long period of time, the tire is mechanically destroyed and also thermally decomposed to such an extent that the tire can no longer be repaired. Furthermore, the irregular rotation of the collapsed tire frequently produces a separation of the tire body from the tire-mounting rim and a lateral turning of the vehicle. Both of these conditions are very dangerous for a driver.
In order to eliminate the above-mentioned disadvantages of the conventional pneumatic tires, various attempts have been made to obtain improvements of the pneumatic tires. For example, in one attempt, the tire body was firmly fixed to the tire-mounting rim so that the tire could not be removed from the rim even when the tire was punctured during running.
In another attempt, the tire was provided with a supporting member which extended from the rim into the air chamber so as to support the tread portion when the tire was punctured and as a result, collapsed.
In still another attempt, the tire body was mounted on the rim in such a manner that the tire could be firmly combined with the rim. Accordingly, the rim could, at its flange, support the load applied to the vehicle during running or turning.
In a further attempt, the sidewall portions of the tire were made of a highly rigid material so as to prevent the tire from collapsing when the tire was punctured.
In a still further attempt, in order to increase the rigidity of the sidewall portion, an elastic reinforcing member having a high stiffness was arranged in the tire. This reinforcing member extended from the bead portion to the tread crown portion through the sidewall portion, and was effective for preventing the tire from collapsing when it was punctured.
The above-mentioned attempts are each effective for improving the steering stability, handling stability and durability of a tire for four-wheel vehicles when the tire is punctured, but are not satisfactory for improving such characteristics of a tire for two-wheel vehicles, that is, motorcycles. Even in the case where the four-wheel vehicle is cornering, the tire inclines in a small angle. Therefore, the camber angle provided to the tire is within a narrow range. Accordingly, when the four-wheel vehicle makes a turn, the lateral force applied to the tire due to the cambering force is very small. That is, the lateral force consists essentially of only a cornering force. However, when the motorcycle makes a turn, it is necessary to incline the tire relative to the vertical direction by a large camber angle. This turn of the motorcycle in the inclined condition produces a large lateral cambering force together with the cornering force exerted on the tire. This great lateral force should be absorbed by the tire even when the tire is punctured for the safety of the driver. Accordingly, the above-mentioned attempts which are effective for improving the tires of four-wheel vehicles, are not always satisfactory for improving the tires of motorcycles. Generally speaking, a run flat tire for motorcycles is required to have the following features.
1. The apparent shape and configuration of the tire are similar to those of conventional tires.
2. Even when the pneumatic pressure of the air chamber of the tire is reduced to atmospheric pressure, the tire can be partly deformed but not caused to completely collapse.
3. Even when the tire is punctured, the punctured tire does not cause the motorcycle to have a reduced steering stability and handling stability not only in straight running but, also, in zigzag running and cornering.
4. Even if the tire is punctured while the motorcycle is running at a high speed of from about 150 to 200 km/hour, the irregular rotation of the punctured tire does not cause the motorcycle body to produce undesirable vibrations and the motorcycle can be stopped with high handling stability after running a certain distance.
5. Even after the tire is punctured, the motorcycle can still run for several hundred kilometers at a velocity of from 90 to 100 km/hour.
6. The tire is provided with means for preventing separation of the tire from the tire-mounting rim when the tire is punctured.
7. When the tire is punctured, the tire can be completely repaired for reuse and such repaired tire will have a high velocity running durability similar to that of the non-punctured tire.
8. The tire is a tubeless tire having a high durability.
9. Since the motorcycle is supported by only two tires, the tires can stably support the motorcycle during straight running.
10. The tire can be easily manufactured by a mass production process at a high rate of productivity and low cost.
European Patent Application Publication No. 0000589 discloses a run flat tire for motorcycles having a high handling stability, cornering stability and durability even when the tire is punctured during running and the pneumatic pressure inside of the tire becomes equal to atmospheric pressure. The run flat tire has a pair of sidewall-reinforcing layers each comprising (1) an elastic filler extending from an end location adjacent to a bead core in a bead portion to the other end location in a tread portion through a sidewall portion of the tire and (2) at least one reinforcing ply, which extends along one side surface of the elastic filler toward the tread portion, is turned up around the end of the elastic filler in the tread portion, and then, further extends along the other side surface of the elastic filler toward the bead portion. This type of run flat tire is effective for satisfying most of the above-mentioned features.
However, the process for incorporating the reinforcing ply into a body of the elastic filler comprises the three steps of adhering half of the reinforcing ply onto a side surface of the elastic filler, turning the reinforcing ply around the end of the elastic filler and, then, adhering the other half of the reinforcing ply to the other side surface of the elastic filler. That is, the process of incorporating the reinforcing ply into the elastic filler requires a large amount of labor and time and results in a high cost of the resultant tire.
Moreover, in the above-mentioned type of the tire, the sidewall reinforcing layer consisting of the reinforcing ply and the elastic filler, each of which has a relatively high rigidity and hardness, naturally exhibits a higher rigidity and hardness than those of the matrix part of the tire surrounding the sidewall reinforcing layer. Especially, in the above-mentioned type of the sidewall reinforcing layer, since the reinforcing ply is turned around the end portion of the elastic filler, the rigidity and hardness, and particularly the resistance to deformation, of a part of the tread portion in which the end portion of the sidewall reinforcing layer is embedded are remarkably different from those of the part of the tread portion adjacent to the end portion of the sidewall reinforcing layer. When the motorcycle runs in a zigzag movement, the above-mentioned remarkable difference in rigidity, hardness and resistance to deformation creates irregular discontinuous forces on a handle of the motorcycle. The irregular discontinuous forces cause the handling stability of the motorcycle to be poor and, also, the rider on the motorcycle to feel uncomfortable.
Under these circumstances, it is desired to provide a pneumatic run flat tire having none of the defects mentioned above.